For surveying a target point, a multiplicity of geodetic methods and geodetic apparatuses have been known since ancient times. In this case, distance and angle from a measuring apparatus to the target point to be surveyed are recorded and, in particular, the location of the measuring apparatus together with reference points possibly present are detected as spatial standard data.
One generally known example of such surveying apparatuses or geodetic apparatuses is a theodolite, a tachymeter or a total station, which is also designated as electronic tachymeter or computer tachymeter. One such geodetic measuring device from the prior art is described in the publication document EP 1 686 350, for example. Such apparatuses have electrical-sensor-based angle and distance measuring functions that permit direction and distance to be determined with respect to a selected target. In this case, the angle and distance variables are determined in the internal reference system of the apparatus and, if appropriate, also have to be combined with an external reference system for absolute position determination.
Modern total stations have microprocessors for digital further processing and storage of detected measurement data. The apparatuses generally have a compact and integrated design, wherein coaxial distance measuring systems and also angle measuring elements, computing, control and storage units are usually integrated in an apparatus. Depending on the expansion stage of the total station, means for motorization of the targeting optical unit, for reflectorless distance measurement, for automatic target seeking and target tracking and for remote control of the entire apparatus are integrated. Total stations known from the prior art furthermore have a radio data interface for setting up a radio link to external peripheral components such as e.g. a data acquisition apparatus, which can be designed, in particular, as a handheld data logger, field computer, notebook, minicomputer or PDA. By means of the data interface, it is possible to output measurement data acquired and stored by the total station for external further processing, to read externally acquired measurement data for storage and/or further processing into the total station, to input or output remote control signals for remote control of the total station or a further external component in particular in mobile use in the field, and to transfer control software into the total station.
Depending on the embodiment of the target point to be surveyed, the measurement accuracy achievable during the surveying process varies. If the target point is constituted for example by a target reflector designed specifically for surveying—such as an all-round prism—then it is possible to achieve significantly more accurate measurement results than in the case of a reflectorless measurement for example with respect to a point to be surveyed on a house wall. This is owing to the fact, inter alia, that the emitted optical measurement beam has a planar beam cross section rather than a punctiform beam cross section and, consequently, not only measurement radiation scattered at the target point that is actually to be surveyed is received, but also that from points in the immediate field of view surroundings of the target point which are likewise impinged on by the measurement radiation. By way of example, the roughness of the surface of the point to be surveyed influences the accuracies of reflectorless measurements in a known way.
For sighting or targeting a target point to be surveyed, surveying apparatuses of the generic type have a targeting device (such as a telescope). In one simple embodiment variant, the sighting device is embodied for example as a telescopic sight. Modern apparatuses can additionally have a camera for detecting an image, said camera being integrated into the telescopic sight, wherein the detected image can be displayed in particular as a live image on a display of the total station and/or a display of the peripheral apparatus—such as a data logger—used for remote control.
The coaxial camera (e.g. having a CCD or CMOS area sensor) provided in addition to the direct viewing channel can be arranged in a further image plane provided in the telescope optical unit, for which purpose a coupling-out of a partial light beam via a beam splitter can be provided, such that an image is recordable through the objective by means of the camera.
The optical system or the optical viewing channel of the targeting device contains in particular an objective lens group, a focusing lens group (focusing optical unit) and an eyepiece, which are arranged in this order from the object side. In this case, the focusing optical unit has a position measuring device and a position transmitter (motor).
A prerequisite for accurate sighting of the target is the sharp imaging of the field of view both on the display of the integrated camera and at the eyepiece of the telescope.
In one very simple embodiment, the focusing can be carried out by means of a manual focus—for example an adjusting screw for altering the position of the focusing optical unit. The manual focus, i.e. the manually settable position of the focusing optical unit, is always dependent on the personal sharpness perception of the user and on the observation means (e.g. eyepiece, camera display).
The camera of the sighting device can therefore have an autofocus unit, wherein the adjustment of the focusing optical unit is carried out e.g. by servomotors and is controlled e.g. with the aid of a contrast assessment on a specific array of a CMOS sensor or by means of a phase comparison. Automatic focusing devices for telescopic sights of geodetic apparatuses are known e.g. from DE 19710722, DE 19926706 or DE 19949580. The camera autofocus functions with an evaluation of the light beams arriving on the camera sensor. In the case of a contrast-based autofocus, a position of the focusing optical unit is derived with the contrast maximum found.
In addition—in a technologically simplifying manner—in the context of a distance-based autofocusing functionality, the position of the focusing optical unit can be set depending on the object distance measured by means of an optoelectronic distance measuring device such that a sharp object image arises on the camera sensor or an optical element with targeting marking (in particular reticle or graticule, or plate with cross-hair marking and line markings) arranged in the focusing plane. Said optical element with the image generated in said plane can then be viewed through the eyepiece. During the distance-based autofocusing, a distance measured by the optoelectronic distance measuring device is translated directly into a position of the focusing optical unit. This is done e.g. with the aid of a look-up table, a diagram or a curve in which the correlation between target distances and focusing optical unit positions to be set are stored.
In the surveying apparatus, an additional separate transmitting and receiving channel branch can be provided for the coaxial electronic distance measurement. Moreover, conventional surveying apparatuses in the meantime have an automatic target tracking function (ATR: “Automatic Target Recognition”), for which a further separate ATR light source—e.g. a multimode fiber output, which emits light having a further defined wavelength—and also a specific ATR camera sensor are additionally integrated in the telescope.
By way of example, the construction of generic telescopic sights of geodetic apparatuses is disclosed in the publication documents EP 1 081 459 or EP 1 662 278.
The high complexity of the telescope necessitates a high outlay for the necessary high-precision mounting and alignment of the optical components, which can comprise lenses, prisms and beam splitters. Therefore, an externally controlled, simple calibration is carried out during production, such that an identically sharp image can be seen at the direct-view viewfinder and also on the display—after focusing on the same target firstly on the basis of the manual focus and secondly on the basis of the camera autofocus and/or distance autofocus.
In the context of production, a standardized autofocus reference curve stored on the basis of e.g. polynomial coefficients is used for the target-distance-dependent autofocusing function, which reference curve according to experience or as proven statistically in the majority of apparatuses leads to a best possible focusing result. Said curve assigns the target distance measurable by the optoelectronic distance measuring device to the focusing optical unit position to be set. With a small number of calibration measures, for each apparatus an apparatus-specific focus offset possibly present is determined and then computed as overall offset with the autofocus reference curve.
However, such errors eliminated by the manufacturing calibration do not remain stable over the course of time. In this regard, they are influenced for example by physical vibration (for instance during transport), by temperature influences or by other material properties that change in a time-dictated manner. One possible consequence of this is that after the focusing by means of the distance measuring device or after the focusing by means of the autofocus of the camera either only the image at the direct-view viewfinder of the telescope or only the image on the camera display, or neither of the two images, is represented sharply. Further time expenditure is then necessary in order, by manual refocusing by means of the actuating wheel of the focusing optical unit, to obtain a sharp image either at the direct-view viewfinder of the telescope or on the electronic viewfinder or the display of the camera.